1. Field of the Invention
The present invention relates to a magnetic head, particularly to a magneto-resistive effect type thin film magnetic head, used for writing/reading information to a recording medium in a magnetic disk device used as an external storage device for a computer or the like. This invention also relates to a method for producing such a magneto-resistive effect type thin film magnetic head.
2. Description of the Related Art
An example of conventionally known method for producing a magneto-resistive effect type thin film magnetic head will now be described with reference to FIGS. 1, 2 and 3. In these drawings, (a) denotes an insulation substrate; (b), a lower shield layer; (c), a first gap; (d), a magnetic resistance element; (e), a second gap; (f), an upper shield; (g), a third gap; (h), a back gap; (i), a lower insulation layer between layers; (j), a thin film coil; (k), an upper insulation layer between layers; (l), a coil terminal; (m), an upper magnetic pole; and (n), a coil lead line.
A lower shield layer (b) of a soft magnetic material is formed on an insulation substrate (a) to form a desired pattern of the shield layer. Then, a first gap film (c) of an insulation material is formed on the lower shield layer (b) and thereafter a magnetic resistance element (d) and an electrically conductive lead line (not shown) are formed on the first gap film (c). A second gap film (e) of an insulation material is then formed thereon to complete a reading element.
Then, an upper shield (f) which also functions as a lower magnetic pole is formed so as to have a desired pattern. Thereafter, a third gap film (writing gap) (g) of an insulation material is formed and, simultaneously, a back gap (h) which is served as a window of a magnetic circuit for mutually connecting the lower magnetic pole (f) and an upper magnetic pole (m).
Then, a coil layer (j) is formed and then an upper photoresist insulation layer (k) between layers is formed so as to cover the coil layer (j) in such a manner that the coil layer (j) is insulated from the lower and upper magnetic poles (f) and (m). In this case, the photoresist is first patterned to be a predetermined shape by photolithography. Then, the photoresist pattern is thermally treated at a temperature lower than the heat resistance temperature of the magnetic resistance element (d) to harden the photoresist to form a lower insulation layer (i). Then, the coil layer (j) is formed by plating with copper (Cu) or the like and thereafter the upper insulation layer (k) between layers is formed by hardening it at the same thermal treatment condition of the above.
Finally, the oxidized film on the back gap (h) is removed and the coil terminal (l) is produced by ion etching and thereafter the upper magnetic pole (m) and a coil lead line (n) are formed by plating or the like to complete a writing element.
As mentioned above, in the known method for producing a magneto-resistive effect type thin film magnetic head, the lower insulation layer (i) between layers and the upper insulation layer (k) between layers were formed by hardening the photoresist by a thermal treatment. The treating atmosphere was an inert gas, such as nitrogen gas (N.sub.2), argon gas (Ar), or a vacuum condition, and the treating time was 2 to 3 hours.
However, the insulation layers (i) and (k) which were obtained by a thermal treatment under the condition of an inert gas, such as nitrogen gas (N.sub.2), argon gas (Ar), or a vacuum condition, have a problem in that a crack was easily generated in these insulation layers (i) and (k) during the coil forming process or the upper magnetic pole forming process, especially at the time when an ion milling was performed or when an ultrasonic cleaning was performed in an organic solvent. In addition, in the prior art, if the heat resistance of the magnetic resistance element (d) was relatively low as compared with the thermal treating temperature (for example, 250.degree. to 280.degree. C.), the thermal treating temperature of these insulation layers (i) and (k) should be lowered so as to match the heat resistance of the magnetic resistance element (d). However, in the prior art, if the thermal treating temperature of these insulation layers (i) and (k) was lowered under the condition of the above-mentioned atmosphere, there would be a problem that a crack would more easily be generated in these insulation layers (i) and (k).
That is to say, as shown in FIG. 4, in the case of the insulation layers (i) and (k) which were treated in the vacuum condition, a stress (MPa) in these layers is relatively large and therefore is within a range of the crack generation, even at a stage before the thermal treatment is performed in which the initial film thickness is thin, as indicated by mark .oval-solid. in FIG. 4.
If the insulation layers (i) and (k) were treated in the nitrogen gas (N.sub.2) condition at a temperature of 250.degree. C. for 1.5 hours, a stress (MPa) becomes relatively larger than 2.5 MPa and within a range of the crack generation, if the initial thickness before thermal treatment was larger than 7 .mu.m, as indicated by mark .DELTA. in FIG. 4.